System and method for facilitating a cardiac rhythm disorder diagnosis based on subcutaneous cardiac monitoring data

ABSTRACT

A system and method for facilitating a cardiac rhythm disorder diagnosis based on subcutaneous cardiac monitoring data with the aid of a digital computer are provided. Cutaneous action potentials of a patient are recorded as electrocardiogram (EGC) data over a set time period using a subcutaneous insertable cardiac monitor. A set of R-wave peaks is identified within the ECG data and an R-R interval plot is constructed. A difference between recording times of successive pairs of the R-wave peaks in the set is determined. A heart rate associated with each difference is also determined. The pairs of the R-wave peaks and associated heart rate are plotted as the R-R interval plot. A diagnosis of cardiac disorder is facilitated based on patterns of the plotted pairs of the R-wave peaks and the associated heart rates in the R-R interval plot.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional patent application is a continuation-in-part ofU.S. patent application Ser. No. 16/102,608, filed Aug. 13, 2018,pending, which is a continuation of U.S. Pat. No. 10,045,709, issuedAug. 14, 2018; which is a continuation of U.S. Pat. No. 9,408,551,issued Aug. 9, 2016; which is a continuation-in-part of U.S. Pat. No.9,345,414, issued May 24, 2016; which is a continuation-in-part of U.S.Pat. No. 9,408,545, issued Aug. 9, 2016; which is a continuation-in-partof U.S. Pat. No. 9,700,227, issued Jul. 11, 2017; which is acontinuation-in-part of U.S. Pat. No. 9,730,593, issued Aug. 15, 2017;and further claims priority under 35 U.S.C. § 119(e) to U.S. ProvisionalPatent Application Ser. No. 62/132,497, filed Mar. 12, 2015, and U.S.Provisional Patent Application Ser. No. 61/882,403, filed Sep. 25, 2013,the disclosures of which are incorporated by reference; this presentnon-provisional patent application is also a continuation-in-part ofU.S. patent application Ser. No. 15/832,385, filed Dec. 5, 2017,pending, the disclosure of which is incorporated by reference.

FIELD

This application relates in general to electrocardiographic monitoringand, in particular, to a system and method for facilitating diagnosis ofcardiac rhythm disorders based on subcutaneous cardiac monitoring data.

BACKGROUND

An electrocardiogram (ECG) allows physicians to diagnose cardiacfunction by visually tracing the cutaneous electrical signals (actionpotentials) that are generated by the propagation of the transmembraneionic currents that trigger the depolarization of cardiac fibers. An ECGtrace contains alphabetically-labeled waveform deflections thatrepresent distinct features within the cyclic cardiac activationsequence. The P-wave represents atrial depolarization, which causesatrial contraction. The QRS-complex represents ventriculardepolarization. The T-wave represents ventricular repolarization.

The R-wave is often used as an abbreviation for the QRS-complex. An R-Rinterval spans the period between successive R-waves and, in a normalheart, is 600 milliseconds (ms) to one second long, which respectivelycorrespond to 100 to 60 beats per minute (bpm). The R-wave is thelargest waveform generated during normal conduction and represents thecardiac electrical stimuli passing through the ventricular walls. R-Rintervals provide information that allows a physician to understand at aglance the context of cardiac rhythms both before and after a suspectedrhythm abnormality and can be of confirmational and collaborative valuein cardiac arrhythmia diagnosis and treatment.

Conventionally, the potential of R-R interval context has not been fullyrealized, partly due to the difficulty of presentation in a concise andeffective manner to physicians. For instance, routine ECGs are typicallydisplayed at an effective paper speed of 25 millimeters (mm) per second.A lower speed is not recommended because ECG graph resolution degradesat lower speeds and diagnostically-relevant features may be lost.Conversely, a half-hour ECG recording, progressing at 25 mm/s, resultsin 45 meters of ECG waveforms that, in printed form, is cumbersome and,in electronic display form, will require significant back and forthtoggling between pages of waveforms, as well as presenting voluminousdata transfer and data storage concerns. As a result, ECGs are less thanideal tools for diagnosing cardiac arrhythmia patterns that only becomeapparent over an extended time frame, such as 30 minutes or longer.

R-R intervals have also been visualized in Poincare plots, which graphRR(n) on the x-axis and RR(n+1) on the y-axis. However, a Poincare plotfails to preserve the correlation between an R-R interval and the R-Rinterval's time of occurrence and the linearity of time and associatedcontextual information, before and after a specific cardiac rhythm, arelost. In addition, significant changes in heart rate, particularlyspikes in heart rate, such as due to sinus rhythm transitions to atrialflutter or atrial fibrillation, may be masked or distorted in a Poincareplot if the change occurs over non-successive heartbeats, rather thanover two adjacent heartbeats, which undermines reliance on Poincareplots as dependable cardiac arrhythmia diagnostic tools. Further,Poincare plots cannot provide context and immediate temporal referenceto the actual ECG, regardless of paper speed. Events both prior to andafter a specific ECG rhythm can provide key clinical informationdisclosed in the R-R interval plot that may change patient managementabove and beyond the specific rhythm being diagnosed.

Further, in addition to the information that a physician can understandfrom an R-R interval plot, the P-wave is a critical component ofarrhythmia monitoring and diagnosis performed every day hundreds ofthousands of times across the United States. Without a knowledge of therelationship of these two basic symbols, heart rhythm disorders cannotbe reliably diagnosed. Visualizing both the P-wave and the R-wave allowfor the specific identification of a variety of atrial tachyarrhythmias(also known as supraventricular tachyarrhythmias, or SVTs), ventriculartachyarrhythmias (VTs), and bradycardias related to sinus node andatrioventricular (AV) node dysfunction. These categories are wellunderstood by cardiologists but only accurately diagnosable if theP-wave and the R-wave are visualized and their relationship and behaviorare clear. Visualization of the R-wave is usually readily achievable, asthe R-wave is a high voltage, high frequency signal easily recorded fromthe skin's surface. However, as the ECG bipole spacing and electrodesurface area decreases, even the R-wave can be a challenge to visualize.To make matters of rhythm identification more complicated, surfaceP-waves can be much more difficult to visualize from the surface becauseof their much lower voltage and signal frequency content. P-wavevisualization becomes exacerbated further when the recording bipoleinter-electrode spacing decreases.

Cardiac rhythm disorders may present with lightheadedness, fainting,chest pain, hypoxia, syncope, palpitations, and congestive heart failure(CHF), yet rhythm disorders are often sporadic in occurrence and may notshow up in-clinic during a conventional 12-second ECG. Continuous ECGmonitoring with P-wave-centric action potential acquisition over anextended period is more apt to capture sporadic cardiac events. However,recording sufficient ECG and related physiological data over an extendedperiod remains a significant challenge. For instance, maintainingcontinual contact between ECG electrodes of conventional ambulatorydermal ECG monitors and the skin after a day or two of ambulatory ECGmonitoring has been a problem. Time, dirt, moisture, and otherenvironmental contaminants, as well as perspiration, skin oil, and deadskin cells from the patient's body, can get between an ECG electrode'snon-conductive adhesive and the skin's surface. These factors adverselyaffect electrode adhesion and the quality of cardiac signal recordings.Furthermore, the physical movements of the patient and their clothingimpart various compressional, tensile, bending, and torsional forces onthe contact point of an ECG electrode, especially over long recordingtimes, and an inflexibly fastened ECG electrode will be prone tobecoming dislodged. Moreover, dislodgment may occur unbeknownst to thepatient, making the ECG recordings worthless. Further, some patients mayhave skin that is susceptible to itching or irritation, and the wearingof ECG electrodes can aggravate such skin conditions.

While subcutaneous ECG monitors can perform monitoring for an extendedperiod of time, up to three years, such subcutaneous ECG monitors,because of their small size, have greater problems of demonstrating aclear and dependable P-wave. The issues related to a tiny atrial voltageare exacerbated by the small size of insertable cardiac monitors (ICMs),the signal processing limits imposed upon them by virtue of theirreduced electrode size, and restricted inter-electrode spacing.Conventional subcutaneous ICMs, as well as most conventional surface ECGmonitors, are notorious for poor visualization of the P-wave, whichremains the primary reason that heart rhythm disorders cannot preciselybe identified today from ICMs. Furthermore, even when physiologicallypresent, the P-wave may not actually appear on an ECG because theP-wave's visibility is strongly dependent upon the signal capturingability of the ECG recording device's sensing circuitry. This situationis further influenced by several factors, including electrodeconfiguration, electrode surface areas and shapes, inter-electrodespacing; where the electrodes are placed on or within the body relativeto the heart's atria. Further, the presence or absence of ambient noiseand the means to limit the ambient noise is a key aspect of whether thelow amplitude atrial signal can be seen.

Conventional ICMs are often used after diagnostic measures when dermalECG monitors fail to identify a suspected arrhythmia. Consequently, whena physician is strongly suspicious of a serious cardiac rhythm disorderthat may have caused loss of consciousness or stroke, for example, thephysician will often proceed to the insertion of an ICM under the skinof the thorax. Although traditionally, the quality of the signal islimited with ICMs with respect to identifying the P-wave, the durationof monitoring is hoped to compensate for poor P-wave recording. Thissituation has led to a dependence on scrutiny of R-wave behavior, suchas RR interval (R-wave-to-R-wave interval) behavior, often used as asurrogate for diagnosing atrial fibrillation, a potential cause ofstroke. To a limited extent, this approach has some degree of value.Nevertheless, better recording of the P-wave would result in asignificant diagnostic improvement, not only in the case of atrialfibrillation, but in a host of other rhythm disorders that can result insyncope or loss of consciousness, like VT or heart block.

As mentioned above, the P-wave is the most difficult ECG signal tocapture by virtue of originating in the low tissue mass atria and havingboth low voltage amplitude and relatively low frequency content.Notwithstanding these physiological constraints, ICMs are popular,albeit limited in their diagnostic yield. The few ICMs that arecommercially available today, including the Reveal LINQ ICM,manufactured by Medtronic, Inc., Minneapolis, Minn., the BioMonitor 2(AF and S versions), manufactured by Biotronik SE & Co. KG, Berlin,Germany, and the Abbott Confirm Rx ICM, manufactured by AbbottLaboratories, Chicago, Ill., all are uniformly limited in theirabilities to clearly and consistently sense, record, and deliver theP-wave.

Typically, the current realm of ICM devices use a loop recorder wherecumulative ECG data lasting for around an hour is continuallyoverwritten unless an episode of pre-programmed interest occurs or apatient marker is manually triggered. The limited temporal windowafforded by the recordation loop is yet another restriction on theevaluation of the P-wave, and related cardiac morphologies, and furthercompromises diagnostic opportunities.

For instance, Medtronic's Reveal LINQ ICM delivers long-termsubcutaneous ECG monitoring for up to three years, depending onprogramming. The monitor is able to store up to 59 minutes of ECG data,include up to 30 minutes of patient-activated episodes, 27 minutes ofautomatically detected episodes, and two minutes of the longest atrialfibrillation (AF) episode stored since the last interrogation of thedevice. The focus of the device is more directed to recording durationand programming options for recording time and patient interactionsrather than signal fidelity. The Reveal LINQ ICM is intended for generalpurpose ECG monitoring and lacks an engineering focus on P-wavevisualization. Moreover, the device's recording circuitry is intended tosecure the ventricular signal by capturing the R-wave, and is designedto accommodate placement over a broad range of subcutaneous implantationsites, which is usually sufficient if one is focused on the R-wave givenits amplitude and frequency content, but of limited value in capturingthe low-amplitude, low-frequency content P-wave. Finally, electrodespacing, surface areas, and shapes are dictated (and limited) by thephysical size of the monitor's housing which is quite small, anaesthetic choice, but unrealistic with respect to capturing the P-wave.

Similar in design is the titanium housing of Biotronik's BioMonitor 2but with a flexible silicone antenna to mount a distal electrode lead,albeit of a standardized length. This standardized length mollifies, inone parameter only, the concerns of limited inter-electrode spacing andits curbing effect on securing the P-wave. None of the other factorsrelated to P-wave signal revelation are addressed. Therefore the qualityof sensed P-waves reflects a compromise caused by closely-spaced polesthat fail to consistently preserve P-wave fidelity, with the reality ofthe physics imposed problems of signal-to-noise ratio limitationsremaining mostly unaddressed.

Therefore, a need remains for a way to capture low amplitude cardiacaction potential propagation during long term cardiac monitoring and topresent R-R interval data to physicians to reveal temporally-relatedpatterns as an aid to rhythm abnormality diagnosis.

SUMMARY

R-R interval data is presented to physicians in a format that includesviews of relevant near field and far field ECG data, which togetherprovide contextual information that improves diagnostic accuracy. Thenear field (or short duration) ECG data view provides a “pinpoint”classical view of an ECG at traditional recording speed in a manner thatis known to and widely embraced by physicians. The near field ECG datais coupled to a far field (or medium duration) ECG data view thatprovides an “intermediate” lower resolution, pre- and post-eventcontextual view.

Both near field and far field ECG data views are temporally keyed to anextended duration R-R interval data view. In one embodiment, the R-Rinterval data view is scaled non-linearly to maximize the visualdifferentiation for frequently-occurring heart rate ranges, such that asingle glance allows the physician to make a diagnosis. All three viewsare presented simultaneously, thereby allowing an interpreting physicianto diagnose rhythm and the pre- and post-contextual events leading up toa cardiac rhythm of interest.

The durations of the classical “pinpoint” view, the pre- and post-event“intermediate” view, and the R-R interval plot are flexible andadjustable. In one embodiment, a temporal point of reference isidentified in the R-R interval plot and the ECG data that is temporallyassociated with the point of reference is displayed in the near fieldand far field ECG data views. In a further embodiment, diagnosticallyrelevant cardiac events can be identified as the temporal point ofreference. For clarity, the temporal point of reference will generallybe placed in the center of the R-R interval data to allow pre- andpost-event heart rhythm and ECG waveform data to present in the correctcontext. Thus, the pinpoint “snapshot” and intermediate views of ECGdata with the extended term R-R interval data allow a physician tocomparatively view heart rate context and patterns of behavior prior toand after a clinically meaningful arrhythmia, patient concern or otherindicia, thereby enhancing diagnostic specificity of cardiac rhythmdisorders and providing physiological context to improve diagnosticability.

The ECG data from which the R-R interval plot is created can be obtainedthrough a continuously-recording subcutaneous insertable cardiac monitor(ICM), such as one described in commonly-owned U.S. patent applicationSer. No. 15/832,385, filed Dec. 5, 2017, pending, the disclosure ofwhich is incorporated by reference. The sensing circuitry and thephysical layout of the electrodes are specifically optimized to captureelectrical signals from the propagation of low amplitude, relatively lowfrequency content cardiac action potentials, particularly the P-wavesthat are generated during atrial activation. In general, the ICM isintended to be implanted centrally and positioned axially and slightlyto either the left or right of the sternal midline in the parasternalregion of the chest.

In one embodiment, a system and method for facilitating a cardiac rhythmdisorder diagnosis based on subcutaneous cardiac monitoring data.Cardiac action potentials of a patient over a set time period arerecorded using an implantable housing comprised of a biocompatiblematerial that is suitable for implantation within a living body; atleast one pair of electrocardiographic (ECG) sensing electrodes providedon a ventral surface and on opposite ends of the implantable housingoperatively placed to facilitate sensing in closest proximity to the lowamplitude, low frequency content cardiac action potentials that aregenerated during atrial activation; and electronic circuitry providedwithin the housing assembly comprising a low power microcontrolleroperable to execute under modular micro program control as specified infirmware, an ECG front end circuit interfaced to the microcontroller andconfigured to capture the cardiac action potentials sensed by the pairof ECG sensing electrodes which are output as ECG signals, andnon-volatile memory electrically interfaced with the microcontroller andoperable to continuously store samples of the ECG signals. The storedsamples of the ECG signals as ECG data are retrieved. A set of R-wavepeaks within the ECG data are identified. An R-R interval plot isconstructed, including determining a difference between recording timesof successive pairs of the R-wave peaks in the set; determining a heartrate associated with each difference; and plotting the pairs of theR-wave peaks and associated heart rate as the R-R interval plot.Diagnosis of a cardiac disorder of the patient based on patterns of theplotted pairs of the R-wave peaks and the associated heart rates in theR-R interval plot is facilitated. The foregoing aspects enhance thepresentation of diagnostically relevant R-R interval data, reduce timeand effort needed to gather relevant information by a clinician andprovide the clinician with a concise and effective diagnostic tool,which is critical to accurate arrhythmia and cardiac rhythm disorderdiagnoses.

Custom software packages have been used to identify diagnosticallyrelevant cardiac events from the electrocardiography data, but usuallyrequire a cardiologist's diagnosis and verification. In contrast, whenpresented with a machine-identified event, the foregoing approach aidsthe cardiologist's diagnostic job by facilitating presentation ofECG-based background information prior to and after the identifiedevent.

Still other embodiments will become readily apparent to those skilled inthe art from the following detailed description, wherein are describedembodiments by way of illustrating the best mode contemplated. As willbe realized, other and different embodiments are possible and theembodiments' several details are capable of modifications in variousobvious respects, including time and clustering of events, all withoutdeparting from their spirit and the scope. Accordingly, the drawings anddetailed description are to be regarded as illustrative in nature andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing, by way of example, a single ECG waveform.

FIG. 2 is a graph showing, by way of example, a prior art Poincaré R-Rinterval plot.

FIG. 3 is a flow diagram showing a method for facilitating diagnosis ofcardiac rhythm disorders with the aid of a digital computer inaccordance with one embodiment.

FIG. 4 is a flow diagram showing a routine for constructing anddisplaying a diagnostic composite plot for use in the method of FIG. 3.

FIG. 5 is a flow diagram showing a routine for constructing anextended-duration R-R interval plot for use in the routine of FIG. 4.

FIG. 6 is a diagram showing, by way of example, a diagnostic compositeplot generated by the method of FIG. 3.

FIG. 7 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of sinus rhythm (SR) transitioninginto atrial fibrillation (AF).

FIG. 8 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of 3:1 atrial flutter (AFL)transitioning into SR.

FIG. 9 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of atrial trigeminy.

FIG. 10 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of maximum heart rate in an episodeof AF during exercise.

FIG. 11 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of SR transitioning into AFLtransitioning into AF.

FIG. 12 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of sinus tachycardia andpalpitations that occurred during exercise accompanied by a jump inheart rate.

FIG. 13 is a diagram showing, by way of example, a diagnostic compositeplot for facilitating the diagnosis of bradycardia.

FIG. 14 is a block diagram showing a system for facilitating diagnosisof cardiac rhythm disorders with the aid of a digital computer inaccordance with one embodiment.

FIG. 15 is a diagram showing, by way of example, a subcutaneous P-wavecentric insertable cardiac monitor (ICM) for long termelectrocardiographic monitoring in accordance with one embodiment.

FIGS. 16 and 17 are respectively top and bottom perspective viewsshowing the ICM of FIG. 15.

FIG. 18 is a bottom perspective view showing the ICM of FIG. 15 inaccordance with a further embodiment.

FIGS. 19 and 20 are respectively top and bottom perspective viewsshowing an ICM in accordance with a still further embodiment.

FIG. 21 is a plan view showing further electrode configurations.

FIG. 22 is a functional block diagram showing the P-wave focusedcomponent architecture of the circuitry of the ICM of FIG. 15.

DETAILED DESCRIPTION

A normal healthy cardiac cycle repeats through an expected sequence ofevents that can be visually traced through an ECG. Each cycle startswith cardiac depolarization originating high in the right atrium in thesinoatrial (SA) node before spreading leftward towards the left atriumand inferiorly towards the atrioventricular (AV) node. After a delay inthe AV node, the depolarization impulse transits the Bundle of His andmoves into the right and left bundle branches and Purkinje fibers toactivate the right and left ventricles.

When a rhythm disorder is suspected, diagnostically-relevant arrhythmicevents in the cardiac cycle can often be identified and evaluated withthe assistance of an ECG and R-R interval tachography, such as Poincaréplots. Routine ECG evaluation is primarily focused identifying changesto expected ECG waveform shapes. FIG. 1 is a graph showing, by way ofexample, a single ECG waveform 10. The x-axis represents approximatetime in units of tenths of a second and the y-axis representsapproximate cutaneous electrical signal strength in units of millivolts.By long-standing convention, ECGs are typically printed or displayed atan effective paper speed of 25 millimeters (mm) per second. Although inpractice an ECG may be provided to a physician in traditionalpaper-printed form, in “virtual” electronic display form, or both, theterm “effective paper speed” is nevertheless still widely applied as ametric to normalize the recorded ECG signal to a standardized grid of 1mm squares (omitted for the sake of clarity in FIG. 1), whereby each 1mm horizontal box in the grid corresponds to 0.04 s (40 ms) of recordedtime. Other effective paper speeds, grid sizes and units of display arepossible.

A full ECG consists of a stream of alphabetically-labeled waveforms 10that collectively cover cardiac performance over a period ofobservation. For a healthy patient, within each ECG waveform 10, theP-wave 11 will normally have a smooth, normally upward, positivewaveform that indicates atrial depolarization. The QRS complex 17 willusually follow, often with a downward deflection of a Q-wave 12,followed by a larger upward deflection of an R-wave 13, and beterminated with a downward waveform of the S-wave 14, which arecollectively representative of ventricular depolarization. The T-wave 15will normally be a modest upward waveform, representative of ventricularrepolarization, while the U-wave 16, which is often not directlyobservable, will indicate the recovery period of the Purkinje conductionfibers.

Rhythm disorders often manifest through R-R interval variability and thepatterns formed by R-R intervals over an extended time period areimportant tools in the diagnosis of cardiac rhythm abnormalities. Forexample, atrial fibrillation (AF) is the chaotic firing of the atriathat leads to an erratic activation of the ventricles. AF is initiallydiagnosed by an absence of organized P-waves 11 and confirmed by erraticventricular rates that manifest in an ECG R-R interval plot as acloud-like pattern of irregular R-R intervals due to an abnormalconduction of impulses to the ventricles. There is a Gaussian-likedistribution to these R-R intervals during AF. Similarly, atrial flutter(AFL) is an abnormal heart rhythm in which cardiac impulses travel alongpathways within the right atrium in an organized circular motion,causing the atria to beat faster than and out of sync with theventricles. During AFL, the heart beats quickly, yet with a regularpattern. Although AFL presents in an electrogram (e-gram) as a“sawtooth” pattern, AFL can be confirmed in an ECG by characteristic R-Rinterval patterns that usually manifest as 2:1 atrioventricular (AV)conduction or 4:1 atrioventricular conduction. On occasion, theconduction through the AV node is variable and not fixed.

Conventionally, R-R intervals have been visualized using Poincare plots.FIG. 2 is a graph showing, by way of example, a prior art Poincare R-Rinterval plot 18. The x-axis represents the duration of R-R interval nin units of milliseconds (ms). The y-axis represents the duration of R-Rinterval n+1 also in units of ms. Ordinarily, the x- and y-axes use thesame units, so as to form a trend line 19 along the 45-degree angle.When an R-R interval is equal to the successive R-R interval, as oftenoccurs when heart rhythm is regular, the dot representing the twointervals falls onto the 45-degree trend line 19. Conversely, when anR-R interval has changed since the preceding R-R interval, the dotrepresenting the two intervals falls off the 45-degree trend line 19and, as the difference between successive R-R intervals increases, thedots fall further away from the trend line 19.

The number of dots deviating from the trend line 19 in a Poincare plotcan indicate the frequency of occurrence of irregular heartbeats whencompared to the number of dots on the trend line 19. The distance of thedots to the trend line 19 can approximate the extent of heart ratechange from one heartbeat to the next. However, as heart rate change islimited to only successively-occurring heartbeats, the linearity of timeand associated contextual information over an extended time frame arelost. In addition, significant changes in heart rate, particularlyspikes in heart rate, such as due to sinus rhythm transitions to atrialflutter, may be masked, distorted or even omitted in a Poincare plot ifthe change occurs over non-successive heartbeats. In summary, a Poincareplot is more useful as a mathematical tool than a physiological one, andtherefore a Poincare plot cannot truly represent what the heart is doingserially over time with respect to changes in the heart's normal andabnormal physiology.

Despite the limitations of Poincare plots and related forms of R-Rinterval tachography, R-R interval data when presented in a formatduplicating temporal physiological events remains a key tool thatphysicians can rely upon to identify temporally-related cardiacdysrhythmic patterns. Interpretation of R-R interval data can beassisted by including multiple temporal points of reference and a plotof R-R interval data that comparatively depicts heart rate variabilityin concert with R-R interval data. FIG. 3 is a flow diagram showing amethod 20 for facilitating diagnosis of cardiac rhythm disorders withthe aid of a digital computer in accordance with one embodiment. Themethod 20 can be implemented in software and execution of the softwarecan be performed on a computer, such as further described infra withreference to FIG. 14, as a series of process or method modules or steps.

As a precursor step, the cutaneous action potentials of a patient aremonitored and recorded as ECG data over a set time period (step 21),which can be over a short term or extended time frame. ECG recordation,as well as physiological monitoring, can be provided through variouskinds of ECU-capable monitoring ensembles, including a standardized12-lead ECG setup, such as used for clinical ECG monitoring, a portableHolter-type ECG recorder for traditional ambulatory ECG monitoring, or awearable ambulatory ECG monitor, such as a flexible extended wearelectrode patch and a removable reusable (or single use) monitorrecorder, such as described in commonly-assigned U.S. Pat. No.9,345,414, issued May 24, 2016, the disclosure of which is incorporatedby reference, the latter of which includes an electrode patch andmonitor recorder that are synergistically optimized to captureelectrical signals from the propagation of low amplitude, relatively lowfrequency content cardiac action potentials, particularly the P-waves,generated during atrial activation. Still other forms of ECG monitoringassembles are possible.

Upon completion of the monitoring period, the ECG and any physiologicaldata are downloaded or retrieved into a digital computer, as furtherdescribed infra with reference to FIG. 14, with, for instance, theassistance of a download station or similar device, or via wirelessconnection, if so equipped, and a vector of the downloaded or retrievedECG data is obtained (step 22). In one embodiment, the vector of ECGdata represents a 40-minute (or other duration) time span that is usedin constructing the plot of R-R interval data, although other pre-eventand post-event time spans are possible. Optionally, apotentially-actionable cardiac event within the vector of ECG data canbe identified and the ECG data during, prior to and after the event isselected (step 23). The event could be identified with the assistance ofa software package, such as Holter LX Analysis Software, licensed byNorthEast Monitoring, Inc., Maynard, Mass.; IntelliSpace CardiovascularImage and Information management system, licensed Koninklijke PhilipsN.V., Amsterdam, Netherlands; MoMe System, licensed by InfoBionic,Lowell, Mass.; Pyramis ECG Management, licensed by Mortara InstrumentInc., Milwaukee, Wis.; ICS Clinical Suite, licensed by SpacelabsHealthcare Inc., Snoqualmie, Wash.; or a customized software package.Alternatively, the potentially-actionable cardiac event could beidentified by a physician or technician during review of the ECG data.

To improve diagnosis of heart rate variability, a diagnostic compositeplot is constructed that includes one or more temporal points ofreference into the ECG data, which provide important diagnostic context,and a plot of R-R interval data is constructed based on the vector ofECG data (step 24), as further described infra with reference to FIG. 4.Briefly, both near field and far field contextual views of the ECG dataare constructed and displayed. Both views are temporally keyed to anextended duration R-R interval data view that, in one embodiment, isscaled non-linearly to maximize the visual differentiation forfrequently-occurring heart rate ranges, such that a single glance allowsthe physician to make a diagnosis. All three views are presentedsimultaneously, thereby allowing the interpreting physician to diagnoserhythm and the pre- and post-contextual events leading up to a cardiacrhythm of interest.

In a further embodiment, findings made through interpretation of heartrate variability patterns in the diagnostic composite plot can beanalyzed to form a diagnosis of a cardiac rhythm disorder (step 25),such as the cardiac rhythm disorders listed, by way of example, inTable 1. For instance, the heart rate variability patterns in thediagnostic composite plot could be provided to a system thatprogrammatically detects AF by virtue of looking for the classicGaussian-type distribution on the “cloud” of heart rate variabilityformed in the plot of R-R interval data, which can be corroborated bythe accompanying contextual ECG data. Finally, therapy to addressdiagnosed disorder findings can optionally be programmed into a cardiacrhythm therapy delivery device (step 26), such as an implantable medicaldevice (IMD) (not shown), including a pacemaker, implantablecardioverter defibrillator (ICD), or similar devices.

TABLE 1 Cardiac Rhythm Disorders Normal sinus rhythm Sinus BradycardiaSinus Tachycardia Premature atrial and ventricular beats Ectopic atrialtachycardia Atrial fibrillation Atrial flutter Atrial or ventricularbigeminy, trigeminy or quadrigeminy Sinus Bradycardia Fusion beatsInterpolated ventricular premature beats Intraventricular conductiondelay Junctional rhythm AV Nodal re-entrant tachycardia AV re-entranttachycardia Wolff-Parkinson-White Syndrome and Pre-excitationVentricular tachycardia Accelerated idioventricular rhythm AV Wenckebachblock AV Type II block Sinoatrial block

A diagnostic composite plot is constructed and displayed to helpphysicians identify and diagnose temporally-related cardiac dysrhythmicpatterns. The diagnostic composite plot includes ECG traces from two ormore temporal points of reference and a plot of R-R interval data,although other configurations of ECG data plots when combined with theR-R interval plot will also provide critical information. FIG. 4 is aflow diagram showing a routine 30 for constructing and displaying adiagnostic composite plot for use in the method 20 of FIG. 3. Specificexamples of diagnostic composite plots are discussed in detail infrawith reference to FIGS. 7-13.

In the diagnostic composite plot, R-R interval data is presented tophysicians in a format that includes views of relevant near field andfar field ECG data, which together provide contextual information thatimproves diagnostic accuracy. In a further embodiment, other views ofECG data can be provided in addition to or in lieu of the near field andfar field ECG data views. The near field (or short duration) ECG dataprovides a “pinpoint” classical view of an ECG at traditional recordingspeed in a manner that is known to and widely embraced by physicians.The near field ECG data is coupled to a far field (or medium duration)ECG data view that provides an “intermediate” lower resolution, pre- andpost-event contextual view. Thus, the extended-duration R-R intervalplot is first constructed (step 31), as further described infra withreference to FIG. 5. Optionally, noise can be filtered from the R-Rinterval plot (step 32), which is then displayed (step 33). Noisefiltering can include low-pass or high-pass filtering or other forms ofsignal processing, including automatic gain control, such as describedin commonly-assigned U.S. patent application Ser. No. 14/997,416, citedsupra.

Rhythm disorders have different weightings depending upon the contextwith which they occur. In the diagnostic composite plot, the R-Rinterval data view and the multiple views of the ECG data provide thatnecessary context. Effectively, the short and medium duration ECG datathat accompanies the extended-duration R-R interval plot represents theECG data “zoomed” in around a temporal point of reference identified inthe center (or other location) of the R-R interval plot, therebyproviding a visual context to the physician that allows temporalassessment of cardiac rhythm changes in various complementary views ofthe heart's behavior. The durations of the classical “pinpoint” view,the pre- and post-event “intermediate” view, and the R-R interval plotare flexible and adjustable. In one embodiment, the diagnostic compositeplot displays R-R interval data over a forty-minute duration and ECGdata over short and medium durations (steps 34 and 35), such asfour-second and 24-second durations that provide two- and 12-secondsegments of the ECG data before and after the R-R interval plot'stemporal point of reference, which is generally in the center of the R-Rinterval plot, although other locations in the R-R interval plot couldbe identified as the temporal point of reference. The pinpoint“snapshot” and intermediate views of ECG data with the extended term R-Rinterval data comparatively depicts heart rate context and patterns ofbehavior prior to and after a clinically meaningful arrhythmia orpatient concern, thereby enhancing diagnostic specificity of cardiacrhythm disorders and providing physiological context to improvediagnostic ability. In a further embodiment, diagnostically relevantcardiac events can be identified and the R-R interval plot can beconstructed with a cardiac event centered in the middle (or otherlocation) of the plot, which thereby allows pre- and post-event heartrhythm data to be contextually “framed” through the pinpoint andintermediate ECG data views. Other durations, intervals andpresentations of ECG data are possible.

The extended-duration R-R interval plot presents beat-to-beat heart ratevariability in a format that is intuitive and contextual, yet condensed.The format of the R-R interval plot is selected to optimizevisualization of cardiac events in a compressed, yet understandablefield of view, that allows for compact presentation of the data akin toa cardiologists understanding of clinical events. FIG. 5 is a flowdiagram showing a routine 40 for constructing an extended-duration R-Rinterval plot for use in the routine 30 of FIG. 4. The duration of theR-R interval plot can vary from less than one minute to the entireduration of the recording. Thus, a plurality of R-wave peaks is firstselected out of the vector of ECG data (step 41) appropriate to theduration of the R-R interval plot to be constructed. For successivepairs of the R-wave peaks (steps 42-43), the difference between therecording times of the R-peaks is calculated (step 43). Each recordingtime difference represents the length of one heartbeat. The heart rateassociated with the recording time difference is determined by taking aninverse of the recording time difference and normalizing the inverse tobeats per minute (step 44). Taking the inverse of the recording timedifference yields a heart rate expressed in beats per second, which canbe adjusted by a factor of 60 to provide a heart rate expressed in bpm.Calculation of the differences between the recording times and theassociated heart rate continues for all of the remaining pairs of theR-wave peaks (step 44).

The pairings of R-R intervals and associated heart rates are formed intoa two-dimensional plot. R-R intervals are plotted along the x-axis andassociated heart rates are plotted along the y-axis. The range and scaleof the y-axis (heart rate) can be adjusted according to the range andfrequency of normal or patient-specific heart rates, so as to increasethe visual distinctions between the heart rates that correspond todifferent R-R intervals. In one embodiment, the y-axis of the R-Rinterval plot has a range of 20 to 300 beats per minute and R-Rintervals corresponding to heart rates falling extremely outside of thisrange are excluded to allow easy visualization of 99+% of the heart ratepossibilities.

In a further embodiment, they-axis has a non-linear scale that iscalculated as a function of the x-axis (R-R interval), such that:

$y = \left( \frac{x - {\min\mspace{14mu}{bpm}}}{{\max\mspace{14mu}{bpm}} - {\min\mspace{14mu}{bpm}}} \right)^{n}$where x is the time difference, min bpm is the minimum heart rate, maxbpm is the maximum heart rate, and n<1. The non-linear scale of they-axis accentuates the spatial distance between successive heart rateswhen heart rate is low. For example, when n=2, the spatial differencebetween 50 and 60 bpm is 32% larger than the spatial difference between90 bpm and 100 bpm, and 68% larger than the spatial difference between150 bpm and 160 bpm. As a result the overall effect is to accentuate thespatial differences in frequently-occurring ranges of heart rate andde-emphasize the spatial differential in ranges of heart rate where adeviation from norm would have been apparent, thus maximizing thespatial efficiency in data presentation. The goal is to show cardiacevents in a simple, small visual contextual format. Larger scales andlarger formats bely the practical limits of single-page presentationsfor the easy visualization at a glance by the busy physician. The visualdistinctions between the heart rates that correspond to different R-Rintervals stand out, especially when plotted on a non-linear scale.Other y-axis ranges and scales are possible as may be selected bydistinct clinical needs and specific diagnostic requirements.

The diagnostic composite plot includes a single, long range view of R-Rinterval data and a pair of pinpoint ECG data views that together helpto facilitate rhythm disorder diagnosis by placing focused long-termheart rate information alongside short-term and medium-term ECGinformation. Such pairing of ECG and R-R interval data is unique in itsability to inform the physician of events prior to, during and after acardiovascular event. FIG. 6 is a diagram showing, by way of example, adiagnostic composite plot 50 generated by the method 30 of FIG. 3. Notethat the diagnostic composite plot can be tailored to include more thanone view of R-R interval data and as many views of contextual ECG dataas needed. In a further embodiment, a background information plotpresenting an extended far field of related information can be included,such as activity amount, activity intensity, posture, syncope impulsedetection, respiratory rate, blood pressure, oxygen saturation (SpO₂),blood carbon dioxide level (pCO₂), glucose, lung wetness, andtemperature. Other forms of background information are possible. In astill further embodiment, background information can be layered on topof or keyed to the diagnostic composite plot 50, particularly at keypoints of time in the R-R interval data plot, so that the contextprovided by each item of background information can be readily accessedby the reviewing physician.

The diagnostic composite plot 50 includes an ECG plot presenting a nearfield (short duration) view 51, an ECG plot presenting an intermediatefield (medium duration) view 52, and an R-R interval data plotpresenting a far field (extended duration) view 53. The three views 51,52, 53 are juxtaposed alongside one other to allow quick back and forthreferencing of the full context of the heart's normal and abnormalphysiology. Typically, a temporal point of reference, which could be adiagnostically relevant cardiac event, patient concern or other indicia,would be identified and centered on the x-axis in all three views. Theplacement of the temporal point of reference in the middle of all threex-axes enables the ECG data to be temporally keyed to the R-R intervaldata appearing in the center 60 of the R-R interval data view 53, with anear field view 51 of an ECG displayed at normal (paper-based) recordingspeed and a far field view 52 that presents the ECG data occurringbefore and after the center 60. As a result, the near field view 51provides the ECG data corresponding to the R-R interval data at thecenter 60 (or other location) in a format that is familiar to allphysicians, while the intermediate field view 52 enables presentation ofthe broader ECG data context going beyond the borders of the near fieldview 51. In a further embodiment, the center 60 can be slidably adjustedbackwards and forwards in time, with the near field view 51 and the farfield view 52 of the ECG data automatically adjusting accordingly tostay in context with the R-R interval data view 51. In a still furtherembodiment, multiple temporal points of reference can be identified witheach temporal point of reference being optionally accompanied by one ormore dedicated sets of ECG data views.

The collection of plots are conveniently arranged close enough to oneanother to facilitate printing on a single page of standard sized paper(or physical paper substitute, such as a PDF file), although otherlayouts of the plots are possible. The far field view 53 is plotted withtime in the x-axis and heart rate in the y-axis. The R-R intervals arecalculated by measuring the time occurring between successive R-wavepeaks. In one embodiment, the far field view 53 presents R-R intervaldata (expressed as heart rate in bpm) that begins about 20 minutes priorto and ends about 20 minutes following the center 60, although otherdurations are possible.

The near field view 51 and intermediate field view 52 present ECG datarelative to the center 60 of the far field view 53. The near field view51 provides a pinpoint or short duration view of the ECG data. In oneembodiment, the near field view 51 presents ECG data 55 that beginsabout two seconds prior to and ends about two seconds following thecenter 60, although other durations are possible. The intermediate fieldview 52 provides additional contextual ECG information allowing thephysician to assess the ECG itself and gather a broader view of therhythm before and after a “blow-up” of the specific arrhythmia ofinterest. In one embodiment, the intermediate field view 52 presents ECGdata 56 that begins about 12 seconds prior to and ends about 12 secondsfollowing the center 60, although other durations are possible. Forconvenience, the eight-second interval of the ECG data 56 in theintermediate field view 52 that makes up the ECG data 56 in the nearfield view 51 is visually highlighted, here, with a surrounding box 57.In addition, other views of the ECG data, either in addition to or inlieu of the near field view 51 and the far field view 52 are possible.Optionally, an ECG plot presenting an extended far field view 54 of thebackground information can be included in the diagnostic composite plot50. In one embodiment, the background information is presented asaverage heart rate with day and night periods 58 alternately shadedalong the x-axis. Other types of background information, such asactivity amount, activity intensity, posture, syncope impulse detection,respiratory rate, blood pressure, oxygen saturation (SpO₂), blood carbondioxide level (pCO₂), glucose, lung wetness, and temperature, arepossible.

Examples of the diagnostic composite plot as applied to specific formsof cardiac rhythm disorders will now be discussed. These examples helpto illustrate the distinctive weightings that accompany different formsof rhythm disorders and the R-R interval and ECG waveform deflectioncontext with which they occur. FIG. 7 is a diagram showing, by way ofexample, a diagnostic composite plot 70 for facilitating the diagnosisof sinus rhythm (SR) transitioning into AF. SR is indicated through thepresence of a reasonably steady baseline, but with subsidiary lines ofpremature beats and their compensatory pauses. SR manifests as ashadowing 71 of a high heart rate line and a low heart rate line. AF ischaracterized by irregular heartbeats with a somewhat random variationof R-R intervals, although within a limited range and concentrating in aGaussian-like distribution pattern around a mean that varies over time.Although AF can be diagnosed by viewing a near field view 51 of ECG datashowing heartbeats with reversed P-wave and irregular R-R intervals,this approach may be unclear when viewing “snippets” of ECG data,especially when associated with poor quality ECG signals. The presenceof AF can also be confirmed through a far field view 53 of R-R intervaldata, in which the R-R intervals assume superficially appearingdisorganized, spread-out and decentralized scattered cloud 72 along thex-axis, in comparison to a concentrated, darkened line typical of a moreorganized cardiac rhythm.

FIG. 8 is a diagram showing, by way of example, a diagnostic compositeplot 80 for facilitating the diagnosis of 3:1 atrial flutter (AFL)transitioning into SR with frequent premature ectopic atrial beats. Inthe initial part of the R-R interval plot, the R-R intervals have adiscernible aggregated line in the middle of the cloud 81 when therhythm has yet to stabilize into a set pattern, not quite AF and notquite AFL. Immediately thereafter, a dense line representing firm 3:1atrial flutter stabilizes the rhythm prior to the transition into SRassociated with the presence of two seesawing baselines that result fromfrequent atrial ectopy causing short coupling intervals and thencompensatory long coupling intervals. SR is indicated by the middle ofthe three lines with a low heart rate line consistent with thecompensatory pause (long coupling interval) and a high heart rate linewith the shortest coupling interval representing the series of atrialpremature beats 82, and thus, at a faster heart rate.

FIG. 9 is a diagram showing, by way of example, a diagnostic compositeplot 90 for facilitating the diagnosis of atrial trigeminy. Atrialtrigeminy is characterized by three heartbeat rates appearingintermittently yet reasonably regularly. Although atrial trigeminy canbe diagnosed by viewing a near field view 51 of ECG data, the pattern issignificantly more recognizable in a far field view 53 of R-R intervaldata, in which a repeating pattern of three distinct heartbeat lines arepersistently present and clearly visible 91. This view also provides thephysician with a qualitative feel for the frequency of the eventtroubling the patient that is not discernible from a single ECG strip.

FIG. 10 is a diagram showing, by way of example, a diagnostic compositeplot 100 for facilitating the diagnosis of maximum heart rate in anepisode of AF during exercise. In a far field view 50 of R-R intervaldata, AF manifests through a dispersed cloud of dots (Gaussian-likedistribution) without a discernible main heart rate line representingregular heartbeats 101. Under exercise, the maximum heartbeat can belocated by an increase in heart rate clustered about the cloud 102. Inaddition, individual dots above the 200 bpm range throughout the entire40-minute range indicates the maximum heart rate during exercise. Thevery rapid rise in heart rate can be critical to patient management, assuch bumps in rate by exercise can prove serious and even triggercardiac arrest. Their very presence is easily visualized in the R-Rinterval data plot, thereby allowing the physician to alter therapysufficiently to control such potentially damaging rises in heart rate.

FIG. 11 is a diagram showing, by way of example, a diagnostic compositeplot 110 for facilitating the diagnosis of SR transitioning into AFLtransitioning into AF. In a far field view 53 of R-R interval data, SRmanifests as an uneven main heart rate line with a fluctuating height111. At the onset of AFL, the main heart rate line breaks away at alower heart rate than the SR main heart rate line 112. The episode ofAFL further evolves into AF as characterized by a dispersed cloud ofirregular heartbeats without concentrated heart rate lines 113. Thisview provides critical information to the physician managing AF patientsin that, at a glance, the view provides data that tells the physicianthat the patient's AF may be the consequence of AFL. Such knowledge mayalter both drug and procedure therapies, like catheter ablation detailsof intervention.

FIG. 12 is a diagram showing, by way of example, a diagnostic compositeplot 120 for facilitating the diagnosis of sinus tachycardia andpalpitations that occurred during exercise accompanied by a jump inheart rate. In a far field view 50 of R-R interval data, sinustachycardia is indicated by the presence of a baseline heart rate ofabout 60 bpm 121 that spikes up to around 100 bpm 122 and graduallyslopes down with a wide tail 123, reflecting a sharp rise of heart ratesfollowed by a gradual decline. The associated ECG data in the near fieldand intermediate field views (not shown) can confirm the rhythm as sinusrhythm and a normal response to exercise. This rhythm, althoughsuperficially obvious, was associated with symptoms of palpitations anddemonstrates a sensitivity to heart rate fluctuations, rather than asensitivity to an arrhythmia. This common problem is often dismissed asmerely sinus tachycardia, rather than recognizing the context of achanging rate that generated the patient's complaint, a problem, visibleonly in the R-R interval data plot.

FIG. 13 is a diagram showing, by way of example, a diagnostic compositeplot 90 for facilitating the diagnosis of bradycardia during sleep and aR-R interval pattern characteristic of sleep. Bradycardia refers to aresting heart rate of under 60 bpm. Bradycardia during sleep is oftentempered with occasional spikes of rapid heart rate, which can be asecondary compensatory response to dreaming, snoring or sleep apnea. Ina far field view 50 of R-R interval data, bradycardia manifests as thepresence of a base line heart rate in the range of about 50 bpm 131,coupled with multiple spikes of dots 132 representing intermittentepisodes of elevated heart rate. Such elevations in heart rate during apre-dominantly slower rate may be signs of a cardio-respiratorydisorder. Still other applications of the diagnostic composite plot 80are possible.

The diagnostic composite plots are a tool used by physicians as part ofa continuum of cardiac care provisioning that begins with ECGmonitoring, continues through diagnostic overread and finally, ifmedically appropriate, concludes with cardiac rhythm disorder treatment.Each of these steps involve different physical components thatcollaboratively allow physicians to acquire and visualize R-R intervaland ECG data in a way that accurately depicts heart rate variabilityover time. FIG. 14 is a block diagram showing a system 140 forfacilitating diagnosis of cardiac rhythm disorders with the aid of adigital computer 150 in accordance with one embodiment. Each diagnosticcomposite plot 151 is based on ECG data 166 that has either beenrecorded by a conventional electrocardiograph (not shown) or retrievedor obtained from some other type of ECG monitoring and recording device.Following completion of the ECG monitoring, the ECG data is assembledinto a diagnostic composite plot 151, which can be used by a physicianto diagnosis and, if required, treat a cardiac rhythm disorder, or forother health care or related purposes.

Each diagnostic composite plot 151 is based on ECG data 166 that hasbeen recorded over a period of observation, which can be for just ashort term, such as during a clinic appointment, or over an extendedtime frame of months. ECG recordation and, in some cases, physiologicalmonitoring can be provided through various types of ECG-capablemonitoring ensembles, including a standardized 12-lead ECG setup (notshown), such as used for clinical ECG monitoring, a portable Holter-typeECG recorder for traditional ambulatory ECG monitoring (also not shown),or a wearable ambulatory ECG monitor.

One form of ambulatory ECG monitor 142 particularly suited to monitoringand recording ECG and physiological data employs an electrode patch 143and a removable reusable (or single use) monitor recorder 144, such asdescribed in commonly-assigned U.S. patent application Ser. No.14/997,416, cited supra. The electrode patch 143 and monitor recorder144 are synergistically optimized to capture electrical signals from thepropagation of low amplitude, relatively low frequency content cardiacaction potentials, particularly the P-waves generated during atrialactivation. The ECG monitor 142 sits centrally (in the midline) on thepatient's chest along the sternum 169 oriented top-to-bottom. The ECGmonitor 142 interfaces to a pair of cutaneous electrodes (not shown) onthe electrode patch 143 that are adhered to the patient's skin along thesternal midline (or immediately to either side of the sternum 169). TheECG monitor 142 has a unique narrow “hourglass”-like shape thatsignificantly improves the ability of the monitor to be comfortably wornby the patient 141 for an extended period of time and to cutaneouslysense cardiac electric signals, particularly the P-wave (or atrialactivity) and, to a lesser extent, the QRS interval signals in the ECGwaveforms indicating ventricular activity.

The electrode patch 143 itself is shaped to conform to the contours ofthe patient's chest approximately centered on the sternal midline. Tocounter the dislodgment due to compressional and torsional forces, alayer of non-irritating adhesive, such as hydrocolloid, is provided atleast partially on the underside, or contact, surface of the electrodepatch, but only on the electrode patch's distal and proximal ends. Tocounter dislodgment due to tensile and torsional forces, a strain reliefis defined in the electrode patch's flexible circuit using cutoutspartially extending transversely from each opposite side of the flexiblecircuit and continuing longitudinally towards each other to define in‘S’-shaped pattern. In a further embodiment, the electrode patch 143 ismade from a type of stretchable spunlace fabric. To counter patientbending motions and prevent disadhesion of the electrode patch 143, theoutward-facing aspect of the backing, to which a (non-stretchable)flexible circuit is fixedly attached, stretches at a different rate thanthe backing's skin-facing aspect, where a skin adhesive removablyaffixes the electrode patch 143 to the skin. Each of these componentsare distinctive and allow for comfortable and extended wear, especiallyby women, where breast mobility would otherwise interfere with ECGmonitor use and comfort. Still other forms of ECG monitoring andrecording assembles are possible.

When operated standalone, the monitor recorder 142 senses and recordsthe patient's ECG data 166 and physiological data (not shown) into amemory onboard the monitor recorder 144. The recorded data can bedownloaded using a download station 145, which could be a dedicateddownload station 145 that permits the retrieval of stored ECG data 166and physiological data, if applicable, execution of diagnostics on orprogramming of the monitor recorder 144, or performance of otherfunctions. To facilitate physical connection with the download station145, the monitor recorder 144 has a set of electrical contacts (notshown) that enable the monitor recorder 144 to physically interface to aset of terminals 148. In turn, the download station 145 can be operatedthrough user controls 149 to execute a communications or data downloadprogram 146 (“Download”) or similar program that interacts with themonitor recorder 144 via the physical interface to retrieve the storedECG data 166. The download station 145 could alternatively be a server,personal computer, tablet or handheld computer, smart mobile device, orpurpose-built device designed specific to the task of interfacing with amonitor recorder 144. Still other forms of download station 145 arepossible. In a further embodiment, the ECG data 166 from the monitorrecorder 144 can be offloaded wirelessly.

The ECG data 166 can be retrieved from the download station 145 using acontrol program 157 (“Ctl”) or analogous application executing on apersonal digital computer 156 or other connectable computing device, viaa hard wired link 158, wireless link (not shown), or by physicaltransfer of storage media (not shown). The personal digital computer 156may also execute middleware (not shown) that converts the ECG data 166into a format suitable for use by a third-party post-monitoring analysisprogram. The personal digital computer 156 stores the ECG data 166 alongwith each patient's electronic medical records (EMRs) 165 in the securedatabase 64, as further discussed infra. In a further embodiment, thedownload station 145 is able to directly interface with other devicesover a computer communications network 155, which could be a combinationof local area and wide area networks, including the Internet or anothertelecommunications network, over wired or wireless connections.

A client-server model can be employed for ECG data 166 analysis. In thismodel, a server 62 executes a patient management program 160 (“Mgt”) orsimilar application that accesses the retrieved ECG data 166 and otherinformation in the secure database 164 cataloged with each patient'sEMRs 165. The patients' EMRs can be supplemented with other information(not shown), such as medical history, testing results, and so forth,which can be factored into automated diagnosis and treatment. Thepatient management program 160, or other trusted application, alsomaintains and safeguards the secure database 164 to limit access topatient EMRs 165 to only authorized parties for appropriate medical orother uses, such as mandated by state or federal law, such as under theHealth Insurance Portability and Accountability Act (HIPAA) or per theEuropean Union's Data Protection Directive. Other schemes and safeguardsto protect and maintain the integrity of patient EMRs 165 are possible.

In a further embodiment, the wearable monitor 142 can interoperatewirelessly with other wearable or implantable physiology monitors andactivity sensors 152, such as activity trackers worn on the wrist orbody, and with mobile devices 153, including smart watches andsmartphones. Wearable or implantable physiology monitors and activitysensors 152 encompass a wide range of wirelessly interconnectabledevices that measure or monitor a patient's physiological data, such asheart rate, temperature, blood pressure, respiratory rate, bloodpressure, blood sugar (with or without an appropriate subcutaneousprobe), oxygen saturation, minute ventilation, and so on; physicalstates, such as movement, sleep, footsteps, and the like; andperformance, including calories burned or estimated blood glucose level.Frequently, wearable and implantable physiology monitors and activitysensors 152 are capable of wirelessly interfacing with mobile devices153, particularly smart mobile devices, including so-called“smartphones” and “smart watches,” as well as with personal computersand tablet or handheld computers, to download monitoring data either inreal-time or in batches through an application (“App”) or similarprogram.

Based on the ECG data 166, physicians can rely on the data as medicallycertifiable and are able to directly proceed with diagnosing cardiacrhythm disorders and determining the appropriate course of treatment forthe patient 141, including undertaking further medical interventions asappropriate. The ECG data 166 can be retrieved by a digital computer 150over the network 155. A diagnostic composite plot 151 that includesmultiple temporal points of reference and a plot of R-R interval data isthen constructed based on the ECG data 166, as discussed in detail suprawith reference to FIG. 3, and displayed or, alternatively, printed, foruse by a physician.

In a further embodiment, the server 159 executes a patient diagnosisprogram 161 (“Dx”) or similar application that can evaluate the ECG data166 to form a diagnosis of a cardiac rhythm disorder. The patientdiagnosis program 161 compares and evaluates the ECG data 166 to a setof medical diagnostic criteria 167, from which a diagnostic overread 162(“diagnosis”) is generated. Each diagnostic overread 162 can include oneor more diagnostic findings 168 that can be rated by degree of severity,such as with the automated diagnosis of atrial fibrillation. If at leastone of the diagnostic findings 168 for a patient exceed a thresholdlevel of tolerance, which may be tailored to a specific client, diseaseor medical condition group, or applied to a general patient population,in a still further embodiment, therapeutic treatment (“Therapy”) toaddress diagnosed disorder findings can be generated and, optionally,programmed into a cardiac rhythm therapy delivery device, such as an IMD(not shown), including a pacemaker, implantable cardioverterdefibrillator (ICD), or similar devices.

In a further embodiment, the ECG data 166 can be recorded using asubcutaneous insertable cardiac monitor. Long-term electrocardiographicand physiological monitoring over a period lasting up to several yearsin duration can be provided through a continuously-recordingsubcutaneous insertable cardiac monitor (ICM), such as one described incommonly-owned U.S. patent application Ser. No. 15/832,385, filed Dec.5, 2017, pending, the disclosure of which is incorporated by reference.FIG. 15 is a diagram showing, by way of example, a subcutaneous P-wavecentric ICM 212 for long term electrocardiographic monitoring inaccordance with one embodiment. The ICM 212 is implanted in theparasternal region 211 of a patient 210. The sensing circuitry andcomponents, compression algorithms, and the physical layout of theelectrodes are specifically optimized to capture electrical signals fromthe propagation of low amplitude, relatively low frequency contentcardiac action potentials, particularly the P-waves generated duringatrial activation. The position and placement of the ICM 212 coupled toengineering considerations that optimize the ICM's sensing circuitry,discussed infra, aid in demonstrating the P-wave clearly.

Implantation of a P-wave centric ICM 212 in the proper subcutaneous sitefacilitates the recording of high quality ECG data with a gooddelineation of the P-wave. In general, the ICM 212 is intended to beimplanted anteriorly and be positioned axially and slightly to eitherthe right or left of the sternal midline in the parasternal region 211of the chest, or if sufficient subcutaneous fat exists, directly overthe sternum. Optimally, the ICM 212 is implanted in a location leftparasternally to bridge the left atrial appendage. However, eitherlocation to the right or left of the sternal midline is acceptable;placement of the device, if possible, should bridge the vertical heightof the heart, which lies underneath the sternum 203, thereby placing theICM 212 in close proximity to the anterior right atrium and the leftatrial appendage that lie immediately beneath.

The ICM 212 is shaped to fit comfortably within the body under the skinand to conform to the contours of the patient's parasternal region 211when implanted immediately to either side of the sternum 203, but couldbe implanted in other locations of the body. In most adults, theproximal end 213 of the ICM 212 is generally positioned below themanubrium 8 but, depending upon patient's vertical build, the ICM 212may actually straddle the region over the manubrium 8. The distal end214 of the ICM 212 generally extends towards the xiphoid process 9 andlower sternum but, depending upon the patient's build, may actuallystraddle the region over or under the xiphoid process 9, lower sternumand upper abdomen.

Although internal tissues, body structures, and tissue boundaries canadversely affect the current strength and signal fidelity of all bodysurface potentials, subsurface low amplitude cardiac action potentials,particularly P-wave signals with a normative amplitude of less than 0.25millivolts (mV) and a normative duration of less than 120 milliseconds(ms), are most apt to be negatively impacted by these factors. Theatria, which generate the P wave, are mostly located posteriorly withinthe thoracic cavity (with the exception of the anterior right atrium,right atrial appendage and left atrial appendage). The majority of theleft atrium constitutes the portion of the heart furthest away from thesurface of the skin on the chest and harbors the atrial tissue mostlikely to be the source of serious arrhythmias, like atrialfibrillation. Conversely, the ventricles, which generate largeramplitude signals, are located anteriorly as in the case of the anteriorright ventricle and most of the anterior left ventricle situatedrelatively close to the skin surface of the central and left anteriorchest. These factors, together with larger size and more powerfulimpulse generation from the ventricles, contribute to the relativelylarger amplitudes of ventricular waveforms.

Nevertheless, both the P-wave and the R-wave are required for thephysician to make a proper rhythm diagnosis from the dozens ofarrhythmias that can occur. Yet, the quality of P-waves is moresusceptible to weakening from distance and the intervening tissues andstructures and from signal attenuation and signal processing than thehigh voltage waveforms associated with ventricular activation. The addedvalue of avoiding further signal attenuation resulting from dermalimpedance makes a subcutaneous P-wave centric ICM even more likely tomatch, or even outperform dermal ambulatory monitors designed toanalogous engineering considerations and using similar sensing circuitryand components, compression algorithms, and physical layout ofelectrodes, such as described in U.S. Pat. No. 9,545,204, issued January217, 20217 to Bishay et al.; U.S. Pat. No. 9,730,593, issued Aug. 15,20217 to Felix et al.; U.S. Pat. No. 9,700,227, issued Jul. 11, 20217 toBishay et al.; U.S. Pat. No. 97,217,433, issued Aug. 1, 20217 to Felixet al.; and U.S. Pat. No. 9,615,763, issued Apr. 11, 20217 to Felix etal., the disclosures of which are incorporated by reference.

The ICM 212 can be implanted in the patient's chest using, for instance,a minimally invasive subcutaneous implantation instrument or othersuitable surgical implement. The ICM 212 is positioned slightly to theright or left of midline, covering the center third of the chest,roughly between the second and sixth ribs, approximately spanningbetween the level of the manubrium 8 and the level of the xiphoidprocess 9 on the inferior border of the sternum 203, depending upon thevertical build of the patient 210.

During monitoring, the amplitude and strength of action potentialssensed by an ECG devices, including dermal ECG monitors and ICMs, can beaffected to varying degrees by cardiac, cellular, extracellular, vectorof current flow, and physical factors, like obesity, dermatitis, lungdisease, large breasts, and high impedance skin, as can occur indark-skinned individuals. Performing ECG sensing subcutaneously in theparasternal region 211 significantly improves the ability of the ICM 212to counter some of the effects of these factors, particularly high skinimpedance and impedance from subcutaneous fat. Thus, the ICM 212exhibits superior performance when compared to conventional dermal ECGmonitors to existing implantable loop recorders, ICMs, and other formsof implantable monitoring devices by virtue of its engineering andproven P-wave documentation above the skin, as discussed in W. M. Smithet al., “Comparison of diagnostic value using a small, single channel,P-wave centric sternal ECG monitoring patch with a standard 3-leadHolter system over 24 hours,” Am. Heart J., March 20217; 2185:67-73, thedisclosure of which is incorporated by reference.

Moreover, the sternal midline implantation location in the parasternalregion 211 allows the ICM's electrodes to record an ECG of optimalsignal quality from a location immediately above the strongestsignal-generating aspects of the atrial. Signal quality is improvedfurther in part because cardiac action potential propagation travelssimultaneously along a north-to-south and right-to-left vector,beginning high in the right atrium and ultimately ending in theposterior and lateral region of the left ventricle. Cardiacdepolarization originates high in the right atrium in the SA node beforeconcurrently spreading leftward towards the left atrium and inferiorlytowards the atrioventricular (AV) node. On the proximal end 213, the ECGelectrodes of the ICM 212 are subcutaneously positioned with the upperor superior pole (ECG electrode) slightly to the right or left of thesternal midline in the region of the manubrium 8 and, on the distal end214, the lower or inferior pole (ECG electrode) is similarly situatedslightly to the right or left of the sternal midline in the region ofthe xiphoid process 9 and lower sternum 203. The ECG electrodes of theICM 212 are placed primarily in a north-to-south orientation along thesternum 203 that corresponds to the north-to-south waveform vectorexhibited during atrial activation. This orientation corresponds to theaVF lead used in a conventional 12-lead ECG that is used to sensepositive or upright P-waves. In addition, the electrode spacing and theelectrodes' shapes and surface areas mimic the electrodes used in theICM's dermal cousin, designed as part of the optimal P-wave sensingelectrode configuration, such as provided with the dermal ambulatorymonitors cited supra.

Despite the challenges faced in capturing low amplitude cardiac actionpotentials, the ICM 212 is able to operate effectively using only twoelectrodes that are strategically sized and placed in locations ideallysuited to high fidelity P-wave signal acquisition. This approach hasbeen shown to clinically outperform more typical multi-lead monitorsbecause of the improved P-wave clarity, as discussed in W. M. Smith etal., cited supra. FIGS. 16 and 17 are respectively top and bottomperspective views showing the ICM 212 of FIG. 1. Physically, the ICM 212is constructed with a hermetically sealed implantable housing 215 withat least one ECG electrode forming a superior pole on the proximal end213 and at least one ECG electrode forming an inferior pole on thedistal end 214.

When implanted, the housing 215 is oriented most cephalad. The housing215 is constructed of titanium, stainless steel or other biocompatiblematerial. The housing 215 contains the sensing, recordation andinterfacing circuitry of the ICM 212, plus a long life battery. Awireless antenna is integrated into or within the housing 215 and can bepositioned to wrap around the housing's internal periphery or locationsuited to signal reception. Other wireless antenna placement orintegrations are possible.

Physically, the ICM 212 has four ECG electrodes 216, 217, 218, 219.There could also be additional ECG electrodes, as discussed infra. TheECG electrodes include two ventral (or dorsal) ECG electrodes 218, 219and two wraparound ECG electrodes 216, 217. One ventral ECG electrode218 is formed on the proximal end 213 and one ventral ECG electrode 219is formed on the distal end 214. One wraparound ECG electrode 216 isformed circumferentially about the proximal end 213 and one wraparoundECG electrode 217 is formed circumferentially about the distal end 214.Each wraparound ECG electrode 216, 217 is electrically insulated fromits respective ventral ECG electrode 218, 219 by a periphery 220, 221.

The four ECG electrodes 216, 217, 218, 219 are programmaticallycontrolled by a microcontroller through onboard firmware programming toenable a physician to choose from several different electrodeconfigurations that vary the electrode surface areas, shapes, andinter-electrode spacing. The sensing circuitry can be programmed, eitherpre-implant or in situ, to use different combinations of the availableECG electrodes (and thereby changing electrode surface areas, shapes,and inter-electrode spacing), including pairing the two ventral ECGelectrodes 216, 217, the two wraparound ECG electrodes 218, 219, or oneventral ECG electrode 216, 217 with one wraparound ECG electrode 218,219 located on the opposite end of the housing 215. In addition, theperiphery 220, 221 can be programmatically controlled to logicallycombine the wraparound ECG electrode 216, 217 on one end of the ICM 212with its corresponding ventral ECG electrode 218, 219 to form a singlevirtual ECG electrode with larger surface area and shape. (Althoughelectronically possible, the two ECG electrodes that are only on one endof the ICM 212, for instance, wraparound ECG electrode 216 and ventralECG electrode 218, could be paired; however, the minimal inter-electrodespacing would likely yield a signal of poor fidelity in mostsituations.)

In a further embodiment, the housing 215 and contained circuitry can beprovided as a standalone ICM core assembly to which a pair of compatibleECG electrodes can be operatively coupled to form a full implantable ICMdevice.

Other ECG electrode configurations are possible. For instance,additional ECG electrodes can be provided to increase the number ofpossible electrode configurations, all of which are to ensure betterP-wave resolution. FIG. 18 is a bottom perspective view showing the ICM212 of FIG. 15 in accordance with a further embodiment. An additionalpair of ventral ECG electrodes 222, 223 are included on the housing'sventral surface. These ventral ECG electrodes 222, 223 are spaced closertogether than the ventral ECG electrodes 218, 219 on the ends of thehousing 215 and a physician can thus choose to pair the two innerventral ECG electrodes 222, 223 by themselves to allow for minimalelectrode-to-electrode spacing, or with the other ECG electrodes 216,217, 218, 219 to vary electrode surface areas, shapes, andinter-electrode spacing even further to explore optimal configurationsto acquire the P-wave.

Other housing configurations of the ICM are possible. For instance, thehousing of the ICM can be structured to enhance long term comfort andfitment, and to accommodate a larger long life battery or more circuitryor features, including physiologic sensors, to provide additionalfunctionality. FIGS. 19 and 20 are respectively top and bottomperspective views showing an ICM 230 in accordance with a still furtherembodiment. The ICM 230 has a housing 31 with a tapered extension 32that is terminated on the distal end with an electrode 34. On a proximalend, the housing 31 includes a pair of ECG electrodes electricallyinsulated by a periphery 37 that include a ventral ECG electrode 33 anda wraparound ECG electrode 34. In addition, a ventral ECG electrode 36is oriented on the housing's distal end before the tapered extension 32.Still other housing structures and electrode configurations arepossible.

In general, the basic electrode layout is sufficient to sense cardiacaction potentials in a wide range of patients. Differences in thoracictissue density and skeletal structure from patient to patient, though,can affect the ability of the sensing electrodes to efficaciouslycapture action potential signals, yet the degree to which signalacquisition is affected may not be apparent until after an ICM has beenimplanted and deployed, when the impacts of the patient's physicalconstitution and his patterns of mobility and physical movement on ICMmonitoring can be fully assessed.

In further embodiments, the electrodes can be configured post-implant toallow the ICM to better adapt to a particular patient's physiology. Forinstance, electrode configurations having more than two sensingelectrodes are possible. FIG. 21 is a plan view showing furtherelectrode configurations. Referring first to FIG. 21(a), a single discECG electrode 40 could be bifurcated to form a pair of half-circle ECGelectrodes 241, 242 that could be programmatically selected or combinedto accommodate a particular patients ECG signal characteristics post-ICMimplant. Referring next to FIG. 21(b), a single disc ECG electrode 245could be divided into three sections, a pair of crescent-shaped ECGelectrodes 246, 247 surrounding a central semicircular ECG electrode 48that could similarly be programmatically selected or combined. Stillother ECG electrode configurations are possible.

ECG monitoring and other functions performed by the ICM 212 are providedthrough a micro controlled architecture. FIG. 22 is a functional blockdiagram showing the P-wave focused component architecture of thecircuitry 280 of the ICM 212 of FIG. 15. The circuitry 280 is poweredthrough the long life battery 21 provided in the housing 215. Operationof the circuitry 280 of the ICM 212 is managed by a microcontroller 281,such as the EFM32 Tiny Gecko 32-bit microcontroller, manufactured bySilicon Laboratories Inc., Austin, Tex. The microcontroller 281 hasflexible energy management modes and includes a direct memory accesscontroller and built-in analog-to-digital and digital-to-analogconverters (ADC and DAC, respectively). The microcontroller 281 alsoincludes a program memory unit containing internal flash memory (notshown) that is readable, writeable, and externally programmable.

The microcontroller 281 operates under modular micro program control asspecified in firmware stored in the internal flash memory. Themicrocontroller 281 draws power from the battery provided in the housing215 and connects to the ECG front end circuit 283. The front end circuit283 measures raw subcutaneous electrical signals using a drivenreference signal that eliminates common mode noise, as further describedinfra.

The circuitry 280 of the ICM 212 also includes a flash memory 82external to the microcontroller 281, which the microcontroller 281 usesfor continuously storing samples of ECG monitoring signal data and otherphysiology, such as respiratory rate, blood oxygen saturation level(SpO₂), blood pressure, temperature sensor, and physical activity, anddevice and related information. The flash memory 82 also draws powerfrom the battery provided in the housing 215. Data is stored in a serialflash memory circuit, which supports read, erase and program operationsover a communications bus. The flash memory 82 enables themicrocontroller 281 to store digitized ECG data. The communications busfurther enables the flash memory 82 to be directly accessed wirelesslythrough a transceiver 285 coupled to an antenna 217 built into (orprovided with) the housing 215. The transceiver 285 can be used forwirelessly interfacing over Bluetooth or other types of wirelesstechnologies for exchanging data over a short distance with a pairedmobile device, including smartphones and smart watches, that aredesigned to communicate over a public communications infrastructure,such as a cellular communications network, and, in a further embodiment,other wearable (or implantable) physiology monitors, such as activitytrackers worn on the wrist or body. Other types of device pairings arepossible, including with a desktop computer or purpose-built bedsidemonitor. The transceiver 285 can be used to offload stored ECGmonitoring data and other physiology data and information, such as bywirelessly transmitting the samples of ECG signals stored in the flashmemory 82 to the download station and any other physiological data tothe download station 145, and for device firmware reprogramming. In afurther embodiment, the flash memory 82 can be accessed through aninductive coupling (not shown), with the accessed samples of the ECGsignals being provided to the download station 145.

The microcontroller 281 includes functionality that enables theacquisition of samples of analog ECG signals, which are converted into adigital representation. In one mode, the microcontroller 281 implementsa loop recorder feature that will acquire, sample, digitize, signalprocess, and store digitized ECG data into available storage locationsin the flash memory 82 until all memory storage locations are filled,after which existing stored digitized ECG data will either beoverwritten through a sliding window protocol, albeit at the cost ofpotentially losing the stored data that was overwritten, if notpreviously downloaded, or transmitted wirelessly to an externalreceiver, such as the download station 145, to unburden the flashmemory. In another mode, the stored digitized ECG data can be maintainedpermanently until downloaded or erased to restore memory capacity. Datadownload or erasure can also occur before all storage locations arefilled, which would free up memory space sooner, albeit at the cost ofpossibly interrupting monitoring while downloading or erasure isperformed. Still other modes of data storage and capacity recovery arepossible.

The circuitry 280 of the ICM 212 can include functionality toprogrammatically select pairings of sensing electrodes when the ICM 212is furnished with three or more electrodes. In a further embodiment,multiple sensing electrodes could be provided on the ICM 212 to providea physician the option of fine-tuning the sensing dipole (or tripole ormultipole) in situ by parking active electrodes and designating anyremaining electrodes inert. The pairing selection can be made remotelythrough an inductive coupling or by the transceiver 285 via, forinstance, a paired mobile device. Thus, the sensing electrodeconfiguration, including number of electrodes, electrode-to-electrodespacing, and electrode size, shape, surface area, and placement, can bemodified at any time during the implantation of the ICM 212.

In a further embodiment, the circuitry 280 of the ICM 212 can include anactigraphy sensor 84 implemented as a 3-axis accelerometer. Theaccelerometer may be configured to generate interrupt signals to themicrocontroller 281 by independent initial wake up and free fall events,as well as by device position. In addition, the actigraphy provided bythe accelerometer can be used during post-monitoring analysis to correctthe orientation of the ICM 212 if, for instance, the ICM 212 has beeninadvertently implanted upside down, that is, with the ICM's housingoriented caudally, as well as for other event occurrence analyses.

In a still further embodiment, the circuitry 280 of the ICM 212 caninclude one or more physiology sensors. For instance, a physiologysensor can be provided as part of the circuitry 280 of the ICM 212, orcan be provided on the electrode assembly 14 with communication with themicrocontroller 281 provided through a circuit trace. The physiologysensor can include an SpO₂ sensor, blood pressure sensor, temperaturesensor, respiratory rate sensor, glucose sensor, airflow sensor,volumetric pressure sensing, or other types of sensor or telemetricinput sources.

In a yet further embodiment, firmware with programming instructions,including machine learning and other forms of artificialintelligence-originated instructions, can be downloaded into themicrocontroller's internal flash memory. The firmware can includeheuristics to signal patient and physician with alerts over healthconditions or arrhythmias of selected medical concern, such as where aheart pattern particular to the patient is identified and the ICM 212 isthereby reprogrammed to watch for a reoccurrence of that pattern, afterwhich an alert will be generated and sent to the physician (or othercaregiver) through the transceiver 285 via, for instance, a pairedmobile device. Similarly, the firmware can include heuristics that canbe downloaded to the ICM 212 to actively identify or narrow down apattern (or even the underlying cause) of sporadic cardiac conditions,for instance, atrial tachycardia (AT), atrial fibrillation (AF), atrialflutter (AFL), AV node reciprocating tachycardia, ventriculartachycardia (VT), sinus bradycardia, asystole, complete heart block, andother cardiac arrhythmias, again, after which an alert will be generatedand sent to the physician (or other caregiver) through the transceiver285. For instance, an alert that includes a compressed ECG digitizedsample can also be wirelessly transmitted by the ICM 212 upon thetriggering of a preset condition, such as an abnormally low heart ratein excess of 170 beats per minute (bpm), an abnormally low heart ratefalling below 30 bpm, or AF detected by onboard analysis of RR intervalvariability by the microcontroller 61. Finally, a similar methodology ofcreating firmware programming tailored to the monitoring and medicaldiagnostic needs of a specific patient (or patient group or generalpopulation) can be used for other conditions or symptoms, such assyncope, palpitations, dizziness and giddiness, unspecified convulsions,abnormal ECG, transient cerebral ischemic attacks and related syndromes,cerebral infarction, occlusion and stenosis of pre-cerebral and cerebralarteries not resulting in cerebral infarction personal history oftransient ischemic attack, and cerebral infarction without residualdeficits, to trigger an alert and involve the physician or initiateautomated analysis and follow up back at the patient's clinic. Finally,in a still further embodiment, the circuitry 280 of the ICM 212 canaccommodate patient-interfaceable components, including an externaltactile feedback device (not shown) that wirelessly interfaces to theICM 212 through the transceiver 285. A patient 210 can press theexternal tactile feedback device to mark events, such as a syncopeepisode, or to perform other functions. The circuitry 280 can alsoaccommodate triggering an external buzzer 267, such as a speaker,magnetic resonator or piezoelectric buzzer, implemented as part of theexternal tactile feedback device or as a separatewirelessly-interfaceable component. The buzzer 67 can be used by themicrocontroller 281 to indirectly output feedback to a patient 210, suchas a low battery or other error condition or warning. Still othercomponents, provided as either part of the circuitry 280 of the ICM 212or as external wirelessly-interfaceable devices, are possible.

In a further embodiment, the ECG front end circuit 283 of the ICM 212measures raw subcutaneous electrical signals using a driven referencesignal, such as described in U.S. Pat. Nos. 9,700,227, 9,7217,433, and9,615,763, cited supra. The driven reference signal effectively reducescommon mode noise, power supply noise and system noise, which iscritical to preserving the characteristics of low amplitude cardiacaction potentials, especially the P wave signals originating from theatria.

The ECG front end circuit 283 is organized into a passive input filterstage, a unity gain voltage follower stage, a passive high passfiltering stage, a voltage amplification and active filtering stage, andan anti-aliasing passive filter stage, plus a reference generator. Thepassive input filter stage passively shifts the frequency response polesdownward to counter the high electrode impedance from the patient on thesignal lead and reference lead, which reduces high frequency noise. Theunity gain voltage follower stage allows the circuit to accommodate avery high input impedance, so as not to disrupt the subcutaneouspotentials or the filtering effect of the previous stage. The passivehigh pass filtering stage includes a high pass filter that removesbaseline wander and any offset generated from the previous stage. Asnecessary, the voltage amplification and active filtering stageamplifies or de-amplifies (or allows to pass-through) the voltage of theinput signal, while applying a low pass filter. The anti-aliasingpassive filter stage provides an anti-aliasing low pass filter. Thereference generator drives a driven reference signal containing powersupply noise and system noise to the reference lead and is connecteddirectly to the patient, thereby avoiding the thermal noise of theprotection resistor that is included as part of the protection circuit.

While the invention has been particularly shown and described asreferenced to the embodiments thereof, those skilled in the art willunderstand that the foregoing and other changes in form and detail maybe made therein without departing from the spirit and scope.

What is claimed is:
 1. A system for facilitating a cardiac rhythmdisorder diagnosis based on subcutaneous cardiac monitoring data,comprising: a subcutaneous insertable cardiac monitor, comprising: animplantable housing for implantation within a living body, theimplantable housing comprised of a biocompatible material; at least onepair of electrocardiographic (ECG) sensing electrodes provided on aventral surface and on opposite ends of the implantable housingoperatively placed to facilitate sensing in closest proximity to lowamplitude, low frequency content cardiac action potentials that aregenerated during atrial activation; and electronic circuitry providedwithin the housing assembly comprising a low power microcontrolleroperable to execute under modular micro program control as specified infirmware, an ECG front end circuit interfaced to the microcontroller andconfigured to capture the cardiac action potentials sensed by the pairof ECG sensing electrodes which are output as ECG signals, andnon-volatile memory electrically interfaced with the microcontroller andoperable to continuously store samples of the ECG signals; and adownload station adapted to retrieve the stored samples of the ECGsignals from the subcutaneous insertable cardiac monitor as ECG data; acomputer comprising a processor and memory within which code forexecution by the processor is stored, the processor configured to:receive the ECG data retrieved by the download station; identify a setof R-wave peaks within the ECG data; construct an R-R interval plot,comprising: determine a difference between recording times of successivepairs of the R-wave peaks in the set and determine a heart rateassociated with each difference; plot the pairs of the R-wave peaks andassociated heart rate as the R-R interval plot; and accentuate in theR-R interval plot spatial differences in frequently-occurring ranges ofthe heart rate and de-emphasize the spatial differences in ranges of theheart rate where a deviation from the frequently-occurring ranges existsusing a non-linear scale for the heart rate; and a display to facilitatediagnosis of a cardiac disorder of the patient based on patterns of theplotted pairs of the R-wave peaks and the associated heart rates in theR-R interval plot.
 2. A system in accordance with claim 1, the processorfurther configured to: form a diagnosis based on heart rate variabilitypatterns in the R-R interval plot.
 3. A system in accordance with claim2, the processor further configured to: detect atrial fibrillation byidentifying a Gaussian-type distribution of heart rate variability inthe duration R-R interval plot.
 4. A system in accordance with claim 1,wherein the processor is configured to form: a background informationplot with the R-R interval plot comprising one or more of activityamount, activity intensity, posture, syncope impulse detection,respiratory rate, blood pressure, oxygen saturation (SpO₂), blood carbondioxide level (pCO₂), glucose, lung wetness, and temperature; andbackground information layered to or keyed with the R-R interval plotcomprising one or more of activity amount, activity intensity, posture,syncope impulse detection, respiratory rate, blood pressure, oxygensaturation (SpO₂), blood carbon dioxide level (pCO₂), glucose, lungwetness, and temperature.
 5. A system in accordance with claim 1,wherein the display displays the R-R interval plot with one or moreviews of at least a portion of the ECG data as contextual data for theR-R interval plot.
 6. A system in accordance with claim 1, wherein eachECG data view comprises one of a near field view providing ECG datacorresponding to the data for the R-wave peak pairs at a center of theR-R interval plot and an intermediate field view providing presentationof additional ECG data than the near field view.
 7. A system inaccordance with claim 1, wherein the set of R-wave peaks is collectedover a period of at least 40 minutes.
 8. A system in accordance withclaim 1, the processor further configured to: select the set of R-wavepeaks by identifying a potential cardiac event within the ECG data andobtaining a portion of the ECG data corresponding to a time during,prior to, and after the potential cardiac event as the set of R-wavepeaks.
 9. A system in accordance with claim 8, the processor furtherconfigured to: represent the potential cardiac event in the R-R intervalplot via a temporal point of reference.
 10. A system in accordance withclaim 9, wherein the display displays the temporal point of reference inthe R-R interval plot in one of a center and another location.
 11. Amethod for facilitating a cardiac rhythm disorder diagnosis based onsubcutaneous cardiac monitoring data, comprising: recording cardiacaction potentials of a patient over a set time period using asubcutaneous insertable cardiac monitor, the subcutaneous insertablecardiac monitor comprising an implantable housing for implantationwithin a living body, the implantable housing comprised of abiocompatible material, the subcutaneous insertable cardiac monitorfurther comprising at least one pair of electrocardiographic (ECG)sensing electrodes provided on a ventral surface and on opposite ends ofthe implantable housing operatively placed to facilitate sensing inclosest proximity to low amplitude, low frequency content cardiac actionpotentials that are generated during atrial activation, the subcutaneousinsertable cardiac monitor further comprising electronic circuitryprovided within the housing assembly, the electronic circuitrycomprising a low power microcontroller operable to execute under modularmicro program control as specified in firmware, an ECG front end circuitinterfaced to the microcontroller and configured to capture the cardiacaction potentials sensed by the pair of ECG sensing electrodes which areoutput as ECG signals, and a non-volatile memory electrically interfacedwith the microcontroller and operable to continuously store samples ofthe ECG signals; and retrieving the stored samples of the ECG signals asECG data; identifying a set of R-wave peaks within the ECG data;constructing an R-R interval plot, comprising: determining a differencebetween recording times of successive pairs of the R-wave peaks in theset; determining a heart rate associated with each difference; plottingthe pairs of the R-wave peaks and associated heart rate as the R-Rinterval plot; and accentuating in the R-R interval plot spatialdifferences in frequently-occurring ranges of the heart rate andde-emphasizing the spatial differences in ranges of the heart rate wherea deviation from the frequently-occurring ranges exists using anon-linear scale for the heart rate; and facilitating diagnosis of acardiac disorder of the patient based on patterns of the plotted pairsof the R-wave peaks and the associated heart rates in the R-R intervalplot.
 12. A method in accordance with claim 11, further comprising:forming a diagnosis based on heart rate variability patterns in the R-Rinterval plot.
 13. A method in accordance with claim 12, furthercomprising: detecting atrial fibrillation by identifying a Gaussian-typedistribution of heart rate variability in the duration R-R intervalplot.
 14. A method in accordance with claim 11, further comprising atleast one of: including a background information plot with the R-Rinterval plot comprising one or more of activity amount, activityintensity, posture, syncope impulse detection, respiratory rate, bloodpressure, oxygen saturation (SpO₂), blood carbon dioxide level (pCO₂),glucose, lung wetness, and temperature; and layering or keyingbackground information with the R-R interval plot comprising one or moreof activity amount, activity intensity, posture, syncope impulsedetection, respiratory rate, blood pressure, oxygen saturation (SpO₂),blood carbon dioxide level (pCO₂), glucose, lung wetness, andtemperature.
 15. A method in accordance with claim 11, furthercomprising: displaying the R-R interval plot with one or more views ofat least a portion of the ECG data as contextual data for the R-Rinterval plot.
 16. A method in accordance with claim 11, wherein eachECG data view comprises one of a near field view providing ECG datacorresponding to the data for the R-wave peak pairs at a center of theR-R interval plot and an intermediate field view providing presentationof additional ECG data than the near field view.
 17. A method inaccordance with claim 11, wherein the set of R-wave peaks is collectedover a period of at least 40 minutes.
 18. A method in accordance withclaim 11, further comprising: selecting the set of R-wave peaks,comprising: identifying a potential cardiac event within the ECG data;and obtaining a portion of the ECG data corresponding to a time during,prior to, and after the potential cardiac event as the set of R-wavepeaks.
 19. A method in accordance with claim 18, further comprising:representing the potential cardiac event in the R-R interval plot via atemporal point of reference.
 20. A method in accordance with claim 19,further comprising: displaying the temporal point of reference in theR-R interval plot in one of a center and another location.